1. Field of the Invention
This invention relates to platform pointing as used in space applications. Particularly, this invention relates to precision pointing measurement for spacecraft platforms in optical and communication payloads.
2. Description of the Related Art
Pointing accuracy is a fundamental problem for any spacecraft design. Virtually every mission requires an ability to accurately point a payload of some type at a distant location. For example, communication satellites require very precise pointing of receive and/or transmit antennas to locations on the ground or to other spacecraft. Similarly, optical payloads for observation satellites require extremely accurate pointing to locations on the Earth. Because of the distances involved in space missions, improving pointing precision is always a desirable objective. An extremely small angular pointing error can result being off target by tens of kilometers. For example, a 0.1 degree angular error will result in pointing off target by more than approximately 60 kilometers. In addition, because a spacecraft structure cannot be made perfectly rigid, payload pointing will be significantly affected by structural vibration and structural deformation on-orbit.
Precise pointing of spacecraft payloads is made more difficult by many factors that can affect the structure of an operating spacecraft, e.g. a satellite placed on orbit. The goal is to achieve a required level precision in pointing a payload from a space based platform. With deployable communication payloads, for example, it may be necessary to measure platform alignment, launch shift and thermal distortion very accurately so that pointing can be adjusted to compensate. In addition to these problems, for optical payloads, high frequency measurements are also sometimes needed for the purpose of compensating payload line of sight (LOS) jitter. In such applications, payload pointing systems and methods are typically required to have relatively high bandwidth in addition to high precision. Simplicity, light weight and easily implemented packaging are also desirable characteristics for any pointing systems. Existing pointing systems can generally be classified into two broad categories, ground based systems and on-board systems.
Ground based systems include a use of ground-based RF and/or optical beacons or landmarks from which optical payloads can register a precisely known location. Pointing accuracy is provided as a relative angular position off the known location of the beacon or landmark. However, beacons are often undesirable for spacecraft because their use entails higher operating costs. The ground beacon must be separately and continuously maintained. In addition, beacons are vulnerable to adversary attacks during a conflict, a particular consideration for any military applications. On the other hand, landmark based solutions introduce significant operational complexities. Fore example, landmarks must be carefully selected, sophisticated imaging processing is required to determine landmarks from images taken by spacecraft payloads, and constant spacecraft and ground spacecraft system link must be maintained.
Although the notion has existed for a time, on-board payload pointing measurement is only an emerging field and no working on-board pointing system has been launched to date. Some developing on-board pointing system technologies include camera based metrology systems, e.g. as developed by the Jet Propulsion Laboratory (JPL) and interferometer based metrology systems, e.g. as developed by JPL, for precision differential path measurements. In addition, Boeing has developed two laser metrology systems for application to large payload structures.
Camera based metrology systems for payload pointing, such as the systems developed by JPL, are relatively expensive and complex. In addition, due to their size and weight, integrating such systems into a spacecraft design is difficult. Furthermore, such systems typically have very low bandwidth, e.g. a highest measurement frequency below 10 Hz. Recently, JPL is also developing an interferometer based laser metrology system for differential path length measurements. However, the system cannot be applied to many pointing problems because spacecraft payload pointing measurement in general is not a path length measurement.
Previous systems have been developed using scanning fan beam laser metrology that have been developed for space based radar applications employing very large radar array structures. Although these systems are ideal for large radar array structures, they are very expensive and far more too complex for general payload pointing applications.
In view of the foregoing, there is a need in the art for systems and methods for providing very precise pointing of space-based platforms in general. Further, there is a need in the art for such systems and methods to provide precision measurements of distortions occurring across spacecraft structures and smaller payload structures. In addition, there is a need for such systems and methods to be less expensive and more precise with high bandwidth measurement. There is also a need for such systems and methods to be simple, light weight and can be easily integrated into a spacecraft design. As detailed hereafter, these and other needs are met by the present invention.